The specification of U.S. Pat. No. 4,574,522 describes the process of growing plant stock in root control bags and some of the advantages to be obtained thereby. The method basically involves the use of a porous fibre bag to confine plant root propagation to essentially the interior of the bag. The fabric must be of a nature which allows root penetration therethrough but restricts the radial expansion of the root after penetration to thereby have a girdling effect on roots which have penetrated the bag wall.
Various advantages have been obtained by using the aforementioned method. These advantages basically relate to the principle that a substantial proportion of root growth remains within the confines of the bag. The girdling effect on roots which penetrate the walls serves to effectively physically prune root growth outside the bag and this tends to promote root branching within the bag. The roots within the bag form a nodule at the point where bag penetration occurs and these nodules serve as a natural carbohydrate reservoir which enhances root growth after the bags have been removed and the plant is transplanted. The very fibrous root growth which occurs within the confines of the bag serves to hold the soil around the roots in a ball during transplantation which enhances the chance of successful transplantation. An important advantage is that with only relatively fine roots passing through the bag walls, removal of the bag and tree from the ground for transplantation elsewhere is a relatively simple task. The point at which the roots pass through the bag walls defines a natural weak structural point and when the bags are removed from the ground roots external to the bag tend to snap off at this weak point permitting easy excavation of the bag and root ball. The aforementioned U.S. specification describes the method of simply lifting the bag straight out of the ground. Another simple method of bag and plant excavation has been developed comprising simply excavating a trench along one side of a row of bag planted trees, and then pushing the trees over to lie in the trench thereby snapping off the roots at their weak point.
The aforementioned advantages rely to a major extent on the integrity of the material from which the bag is made. It has been found in practice that unrestricted root escapes through the bag walls tend to reduce significantly the advantages referred to, to a greater or lesser extent. If even one unrestrained root penetrates the bag wall this root has a tendency to rapidly expand in size and diameter until a major proportion of the plant's nutrient requirement is being supplied through the unrestrained root. FIG. 1 depicts a root control bag where unresticted root penetration has occurred.
Where unrestricted root penetration does occur the excavation of the plant becomes far more difficult in that the unrestricted root must be located and severed before the bag and plant can be removed from the ground. This generally involves digging right around the circumference of the bag, and possibly beneath the bag which is clearly a time consuming and labour intensive task. The aforementioned quick excavation techniques are not effective where unrestricted root penetration has occurred. Also, where unrestricted root penetration occurs the tendency of the roots to branch out within the bag is not as great, and the nodule formations do not occur as readily. Thus, carbohydrate is not retained within the bag to the same extent and secondary root branching does not occur to the same extent.
The problems referred to have the effect of decreasing the survival rate of trees which have been transplanted, or at least significantly limiting the growth of the tree after transplantation if it has been necessary to sever major roots which have penetrated the bag walls. Thus, the problems associated with unrestricted root penetration become compounded. Firstly, the cost of planting the nursery stock within a bag is greater than simply planting stock without a bag because of the cost of the bag and the cost of excavating a hole for the bag size. Secondly, excavation of bag and plant is as difficult as is excavating stock not planted in bags because of the problem of locating and severing the unrestricted root or roots. Thirdly, where unrestricted roots have escaped the root structure within the bag will not have developed to the same extent as root structures where complete root containment has occurred and accordingly the advantages referred to above will not be obtained. Thus, for a greater cost, transplantation stock is produced which is not significantly better than stock produced using traditional ball and burlap methods.
If any weak point exists in the material from which a bag is made there is a reasonable chance that a root will penetrate through that weak point. The aforementioned U.S. specification provides the example of a green ash tree being grown within a root control bag and developing a root structure with approximately 5000 roots wherein unrestricted root growth would have produced only about 150 roots for a similar growth period. Clearly, where such a massively fibrous root growth structure is developed the chance of one of those roots uncovering and penetrating the weak point in the bag is increased. The weak point can occur at any point in the material and it has been found to be particularly prevalent in the lower corners of the bag where the bag wall is stitched to the base of the bag.
It has been found where using root control bags manufactured from SUPAC or DUON non-woven fabric material by Phillips Fibres Corporation the following example results were obtained:
__________________________________________________________________________ TABLE SHOWING THE NUMBER OF ROOT PENETRATION FAILURES IN ROOT CONTROL BAGS MADE FROM PHILLIPS SUPAC MATERIAL (170 g/m.sup.2). FAILURES NOTED WITH AT LEAST QUANTITY PLANT MAJOR ROOT PENETRATION TOTAL VARIETY THROUGH BAG WALL __________________________________________________________________________ 70 ULMUS PROCERA 57% LOUIS VAN HOUTTE 26 ALNUS GLUTINOSA 80% 17 PLATANUS ORIENTALIS 65% 40 MALUS IOENSIS PLENA 57% 48 QUERCUS PALUSTRUS 79% 16 FRAXINUS EXCELSIOR 53% AUREA 17 FRAXINUS RAYWOODI 60% 17 ROBINIA FREESIA 41% __________________________________________________________________________
Clearly these results are unsatisfactory. The problem in developing a material which has operational characteristics superior to that of the aforementioned Phillips material is that the requirements of the material are to some extent contradictory. Firstly, it is necessary that the roots are easily able to penetrate the bag walls, for if penetration does not occur the roots simply circle around within the bag in much the same manner as occurs with solid wall containers. Secondly, once a root has penetrated the bag wall the material must be able to provide the high strength girdling effect necessary to restrict radial expansion of the penetrated root. Thirdly, the material must be substantially free of weak points, and specifically, no one area should be so weak as to present an area which does not exhibit the girdling effect referred to above. The material should also not be subject to deterioration during the expected period of use of the bag in below ground conditions.
It is an object of this invention to provide a root control bag made from a material which satisfies these requirements.